wizard2014,
всё было гораздо проще - строка 13
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
<title> - jsFiddle demo</title>
<script type='text/javascript' src='http://code.jquery.com/jquery-1.10.2.js'></script>
<script type='text/javascript' src="http://code.jquery.com/ui/1.10.4/jquery-ui.js"></script>
<link rel="stylesheet" type="text/css" href="http://code.jquery.com/ui/1.10.4/themes/smoothness/jquery-ui.css">
<script>
$(window).load(function(){
$(".accordion").accordion({
header: "> h3",
active: false,
collapsible: true,
heightStyle: "content"
})
});
</script>
</head>
<body>
<div class="accordion">
<h3>Section 1</h3>
<div>
<div class="accordion">
<h3>Inner Section 1</h3>
<div>
Sed non urna. Donec et ante. Phasellus eu ligula. Vestibulum sit amet
purus. Vivamus hendrerit, dolor at aliquet laoreet, mauris turpis porttitor
velit, faucibus interdum tellus libero ac justo. Vivamus non quam. In
suscipit faucibus urna.
</div>
<h3>Inner Section 2</h3>
<div>
Sed non urna. Donec et ante. Phasellus eu ligula. Vestibulum sit amet
purus. Vivamus hendrerit, dolor at aliquet laoreet, mauris turpis porttitor
velit, faucibus interdum tellus libero ac justo. Vivamus non quam. In
suscipit faucibus urna.
</div>
</div>
</div>
<h3>Section 2</h3>
<div>
Sed non urna. Donec et ante. Phasellus eu ligula. Vestibulum sit amet
purus. Vivamus hendrerit, dolor at aliquet laoreet, mauris turpis porttitor
velit, faucibus interdum tellus libero ac justo. Vivamus non quam. In
suscipit faucibus urna.
</div>
</div>
</body>
</html>